The vestibular system is that part of each inner ear that consists of three semicircular canals, which are head rotation sensors, and two otolith organs: the saccule and utricle. The saccule and utricle are sacs lying within the vestibule of the labyrinth. These sacs act as transducers to transform linear acceleration into signals to be communicated to the central nervous system. Because of their complex three-dimensional anatomical structure, the saccule and utricle are sensitive to linear acceleration in all three directions.
When exposed to an acoustic stimulus of sufficient amplitude, the saccule activates the neck muscles. Thus, one can assess saccular function by exposing the patient to an acoustic stimulus, and observing the output of an electromyograph coupled to the neck muscles. This method of testing saccular function is known in the art as the vestibular evoked myogenic potential (“VEMP”) test.
A difficulty with the VEMP test is that the measured electromyograph signal is noisy and has large measurement variations. As a result, repeated measurements are often required to obtain an accurate result. This makes the VEMP test time-consuming.
In addition, the amplitude of the acoustic stimulus used in the VEMP test is so high that repeated exposure may damage hearing. Typical thresholds for stimulating a response from the saccule in normal humans are on the order of 90-110 dB SPL. Thresholds for stimulating a response from the utricle and the semicircular canals in patients with certain disorders are even higher. Thus, the repeated exposure to loud acoustic stimuli required to overcome the noisy VEMP measurement can contribute to hearing loss.
The VEMP test also exhibits high variability between patients or between different measurements on the same patient. These variations arise because of differences in neck tension, and electrode placement. In addition, some patients experience nerve and muscle fatigue with repeated stimulation.